Even if there is enough "developer" in 30 ml of stock solution, diluting it to weaker than 1+4 reduces the activity of the solution. So you have to use enough stock to make a 1+4 mix to cover the reel in the tank. The pages I mentioned indicate there is an excess of developer as extending the times for multiple rolls indicates. The one shot use is for optimum image quality and consistency as the document indicates. BTW, active developing agent consumption during processing depends on the amount of exposed silver in the roll. A roll of film exposed to room light will likely exhaust a tank full, an unexposed roll not so much.